The present invention relates to a method for producing a hollow body from sheet metal, especially from aluminum or alloys thereof. Known as one such method for producing a hollow body is, for example, the internal high-pressure forming method wherein the workpiece to be formed is inserted in a die suitable for forming and subsequently a liquid or gaseous, i.e. fluid, pressure medium introduced into the die such that it causes the workpiece in the die to be formed by the pressure of the fluid medium, whereby the workpiece is able to conform to the inner wall of the die.
Furthermore, the present invention relates to a hollow body produced by one such method as well as to an intermediate product as may be used for producing one such hollow body by the method in accordance with the invention.
Methods of producing so-called tubular or channel plates, i.e. plates including tubes or coiled tubing, are known in which two flat sheets of aluminum or alloys thereof located one on the other are fixedly joined to each other and the sheet metal portions located inbetween, and not joined to each other, are bulged in a die by introducing a pressure medium to form the channels between the plates. Also known are methods for forming single-ply plates in a die by subjecting them to the pressure of a pressure fluid at one end and stretching them into the cavity of the die (EP-A-0 581 458 and AST Speciality Handbook, Aluminum and Aluminum Alloys, The Materials Information Society, 1993, page 245). These known methods and not always satisfactory, especially when workpieces need to be subjected to a relatively strong change in shape, for example, in cases in which the tubular bodies or extruded sections need to be strongly dilated, particularly where aluminum or alloys thereof are concerned which do not permit forming as good as steel. However, even in the case of steel, strongly forming workpieces fails to be satisfactory with the methods as known.
To increase the inner cross-sections of a workpiece it is known from DE-A-42 32 161 to produce a hollow body in making use of a basic body produced of blanks, welded to each other at the edges, by the combined application of internal high pressure forming and deep drawing. Due to this making it necessary to combine two methods, producing the hollow body is comparitively complicated for one thing. For another, the production means needed for this purpose is extremely complicated and expensive. Furthermore, it is practically not possible to produce very large tanks of aluminum or alloys thereof, such as tanks for motor vehicles, aircraft or ships, due to the insufficient deformation capacity of the aluminum material.
One method of achieving enhanced forming of workpieces of aluminum or an alloy thereof is described in DE-A-195 31 035 in which several flat sheet metal blanks of aluminum or an alloy thereof are abutted edgewise and joined by spot welding into the workpiece to be formed. In this arrangement, individual flat sheet metal blanks are inserted in the workpiece for increasing the volume of the hollow body to be later produced from the workpiece. Subsequently, the workpiece thus configured for forming is inserted in a die and expanded outwards and deformed by the effect of heated oil introduced therein in achieving the hollow body by internal high pressure forming. In this arrangement the joined sheet metal blanks conform to the inner contour of the die by applied internal pressure. However, there are limits to the degree to which the hollow body volume can be expanded as dictated, for one thing, by the stress limit of the solder joints between the individual sheet metal blanks and, for another, the material already expanded becomes consolidated in the region of sharp-edged cross-section transitions of the die due to the extreme deflection of the material in these areas. The increasing strain on the material associated with the increase in strength harbors the risk of cracks occurring, this being the reason why a minimum critical bending radius must not be violated. This is why no acute forming in the corner portions is achievable. Although it is possible with this method to obtain a strong forming of the workpiece in the die in producing a large volume hollow body, however, there are also limits here too, to the size and shape of the hollow body volume to be produced. More particularly, it is not possible with this known method to produce minute forming radii, near to being sharp-edged, and also large hollow bodies in one piece and without flanges extending over the full contour.
In view of these drawbacks and the problems remaining in methods known from prior art, the invention is based on the objective of providing a method and an intermediate product for producing a hollow body of sheet metal, more particularly of aluminum or alloys thereof, which permits with minimum complication configuring a large hollow body volume corresponding to the space available in the fitted condition (for example as in tanks for motor vehicles in the rear axle area thereof) whilst simultaneously achieving very small forming radii and very large forming depths.
In accordance therewith, first a substantially flat sheet metal blank, more particularly of aluminum or an alloy thereof, is provided as the workpiece, although in general any metal material having a suitable forming capacity may be employed as the workpiece material, i.e. light metals through non-alloyed and alloyed hardened steels up to tempered and stainless steels. In this providing step of the method it is possible to determine the shape and size of the sheet metal blank, and by suitably selecting the dimensions of the sheet metal blank to already influence the desired final outer contour of the hollow body to be produced. Working the sheet metal blank is done preferably by known ways and means mechanically by cutting or thermally by flame and laser cutting.
The suitably blanked sheet metal is then folded and/or bent in a subsequent forming process in the region of a wall intermediate element configured integrally with the sheet metal blank. In this arrangement the wall intermediate element forms a section or partial area of the sheet metal blank, i.e. the wall intermediate element is integrated in the sheet metal blank. This internal configuration greatly facilitates handling the sheet metal blank as well as folding and/or bending. Furthermore, this enables the productivity and accuracy of the sheet metal blank to be enhanced, since particularly unlike the method as it reads from DE-A-195 31 035 no individual formed parts need to be soldered together. The hollow bodies to be produced by the method in accordance with the invention in the internal high pressure forming method can thus replace joined, e.g. soldered, welded or beaded constructions of individual sheet metal blanks in avoiding the problems associated with leakage or dynamic fracture in the jointing zone and assuring improved strength properties.
Bending may be done in this step of the method, for example, by roll forming, providing greater freedom for configuration as regards shaping the wall intermediate element. In this arrangement it is also possible to achieve a very sharp-edged bending of the sheet metal in the region of the wall intermediate element in thus making folding possible in several plies, located substantially superimposed. Accordingly, the individual sheet metal plies may be folded, for example, two-ply, three-ply or multi-ply one on the other.
In the case of folding and/or bending the wall intermediate element, it is furthermore possible to relate positioning the edge portions of the sheet metal blank to facilitate subsequent joining thereof. For joining, the edge portions of the sheet metal blank are abutted, preferably with a certain overlap, and joined together, for example, by soldering, welding, such as e.g. mash seam welding or laser beam welding, or also by edging, preferably along the longitudinal extension of the edges of the sheet metal blanks. This step in the method results in a flat hollow body having a closed cross-sectional shape which, by the joining at the edges, is substantially pressure-tight and thus ready for forming in the subsequent internal high-pressure forming step.
In the course of the internal high-pressure forming step a hydraulic or pneumatic pressure medium is introduced into a cavity, defined by the sheet metal blank and the at least one folded wall intermediate element, through a suitable connector, for example, by piping or sockets sealingly joined (e.g. by soldering) in the sheet metal blank or wall intermediate element. By means of the pressure medium the sheet metal blank and the folded wall intermediate element are subsequently subjected to an internal high pressure, as a result of which the multi-ply folded wall intermediate element freely unfolds to its full length in forming the hollow body. Thus, freely shaping the hollow body is achieved in accordance with the invention by unfolding of the wall intermediate element without the workpiece being exposed to any appreciable material strain. Unlike prior art methods the desired shape is not defined by the geometry of the tooling involved in the internal high-pressure forming method, e.g. the die, but by the shape and size of the sheet metal blank and/or wall intermediate element made available at the start of the method.
The gist of the invention is thus to provide a primary increase in volume of the hollow body by unfolding the multi-ply configured wall intermediate element to its extended length without, however, causing any substantial strain in the material. This achieves the major advantage of the full strain capacity of the material still being available for additional localized deepenings due to stretching or deep drawing in subsequent hydroforming, where needed. Due to the method in accordance with the invention, especially when employing materials such as aluminum having a low deformation capacity, stress peaks in the material are avoided so that a hollow body having small forming radii and large forming depths can be configured. Accordingly, the method in accordance with the invention is particularly suitable for producing workpieces produced by lightweight structures thus necessitating the use of the specifically lighter material aluminum, as in the case of e.g.tanks for motor vehicles, aircraft, ships or the like.
The gist of the present invention is likewise based on providing a method and an intermediate product for producing a hollow body which ensures configurations of more complex geometry, higher forming accuracy and strength properties for a reduced workpiece mass in a hollow body.
To achieve the desired final contour of the hollow body to be produced, the unfolding wall intermediate element and/or the sheet metal blank are preferably formed at least sectionwise after being unfolded by an increased internal pressure in the stretching or deep drawing process, for example, by placing the workpiece in a suitable die. Since the sheet metal blank with the wall intermediate element is first merely unfolded by the applied hydraulic or pneumatic internal pressure, and not deep drawn or stretched, the method in accordance with the invention now makes it possible to form the hollow body in a second forming process precisely to the desired shape by internal high pressure. Contrary to prior art forming methods, in which there are limits to shaping the workpiece especially due to the limited forming capacity of the material, these limits are now eliminated by stretching or deep drawing in a step separate from the unfolding of the hollow body, so that in this second forming step even workpieces of complex geometry can be produced. The desired larger volumes of the tanks are thus achieved by the folded sheet metal sections of the wall intermediate element which are first unfolded and then, where necessary, fully or partly stretched or deep drawn.
For shaping the hollow body to be produced, preferably at least one upper and one lower sheet metal section of the blank are inserted superimposed during folding and/or bending. In this arrangement, the sheet metal sections are arranged superimposed preferably in parallel and substantially coplanar, resulting in a substantially closed cross-sectional shape of a flat hollow body. This greatly facilitates subsequent overlapping and joining the edge portions of the sheet metal blank.
To close and seal the workpiece before unfolding by application of the internal high pressure, it is good practice to join the superimposed sheet metal sections at least sectionwise at their faces, e.g. by soldering or welding. In this arrangement the faces of the sheet metal sections of the blank are preferably pressed together so that the individual plies of the folded wall intermediate element abut to advantageously permit joining, e.g. by soldering or welding the faces of the plies of the wall intermediate element to each other and to the faces of the adjoining sheet metal sections.
When the multi-ply arrangement of the wall intermediate element results in an opening between the upper and lower sheet metal section, to facilitate attaining a closed cavity, it is preferred to provide a spacer, such as e.g. of sheet metal, between the superimposed faces of the sheet metal sections in the region of the opening. This enables a spacing or clearance having materialized by the side folding of the multi-ply arrangement wall intermediate element between the sheet metal sections to be bridged and filled to advantage to ensure reliable closure of the cavity.
For face closing the cavity it is furthermore good practice to configure at least one tab-like section at the edge of the upper or lower sheet metal section. After the sheet metal section his been folded or bent into the desired shape, this tab section is folded over at the face of the lower or upper sheet metal section and joining thereto, e.g. by soldering, this too ensuring a tight closure of the faces of the workpiece.
For simple geometry shaping with a cuboidal or rectangular contour, it is preferred to provide the flat sheet metal blank within the scope of step a) of the method with a substantially rectangular flat shape which, on the one hand, is simple to produce whilst ensuring, on the other, facilitated folding or bending of the sheet metal blank. However, other flat shapes of the sheet metal blank are just as possible, such as e.g. round, oval, square or polygonal. Furthermore, appendages or sections protruding outwards beyond the e.g. rectangular flat shape may be configured which to advantage in the scope of step b) of the method are folded inwards in providing the desired unfolding sheet metal sections for a large cavity volume when later unfolded. This configuration of the flat sheet metal blank in various shapes and sizes ensures diversified shaping for the hollow body to be produced.
In pre-forming or forming the hollow body to its final contour, it is good practice when the folded sheet metal blank is inserted into a closable die before or after the unfolding process which, to facilitate inserting the folded sheet metal blank and removal of the formed hollow body, is sectioned and its die cavity preferably corresponding to the desired outer contour of the hollow body to be produced. By flooding the workpiece with a pressure fluid the workpiece receives a high pressure from within and is simultaneously formed to the inner contour of the die simultaneously or after unfolding of the multi-ply configured wall intermediate element. The die ensures additional stability of the workpiece during forming, so that by means of this method even workpieces difficult to form, such as e.g. of aluminum or alloys thereof can now be subjected to strong forming with no problem.
For subjecting the sheet metal blank to a hydraulic or pneumatic internal high pressure, it is good practice to employ as the pressure fluid an oil having ambient temperature or heated oil. In general, however, any other suitable pressure fluid, e.g. water, emulsion, compressed air or the like may be employed for internal high-pressure forming. Using oil, as compared to gases, offers, however, the advantages of it being incompressible and having a substantially higher thermal capacity so that the workpiece can be subjected to stronger forming. In addition, making use of oil avoids the danger of an explosion as could be the case when employing gas mixtures. Using oil, as compared to gases, thus offers substantial advantages as regards a more facilitated handling of the pressure medium. Heating the oil used as the pressure medium, preferably to a temperature of at least 150xc2x0 C., offers furthermore the advantage that during hydroforming of the workpiece to be formed, heat is supplied to a sufficient degree, whereby, especially in forming hollow bodies of aluminum or alloys thereof, a high deformation capacity is retained even when strongly forming into large-volume hollow bodies.
In accordance with a further aspect, the present invention relates to a hollow body comprising at least one wall intermediate element unfolding by an internal high pressure. This hollow body features the advantages already cited in describing the method in accordance with the invention, such as sharp-edged contours, small forming radii and large forming depths for a large hollow body volume and high strength due to production from a single sheet metal blank.
For configuring a stable hollow body having good strength properties, it is preferred that it comprises a substantially axially or rotationally symmetrical shape, permitting, for example, the production of cuboidal, but also oval or polygonal hollow bodies. Configuring such axially symmetrical bodies materializes to advantage by arranging an even number of, for example two, wall intermediate elements in the workpiece to be formed.
To optimally adapt the hollow body volume to the space available in the installed condition, it is of advantage in certain applications when the hollow body comprises a shape dilated substantially partially in one or more planes and directions. In this arrangement the hollow body may assume for example, a conical shape. Achieving this shape is simple, by arranging only one wall intermediate element in the workpiece to be formed.
As evident from that said above, the intermediate product in accordance with the invention for producing a hollow body of sheet metal, more particularly of aluminum or an alloy thereof, comprises a sheet metal blank as well as at least one wall intermediate element configured integrally with the sheet metal blank, and folding and/or bending into several plies located substantially superimposed, results in a sandwich-type structure for the wall intermediate element. Furthermore, edge portions of the sheet metal blank are joined to each other e.g. by soldering, welding, beading or the like so that an intermediate product having a closed cross-sectional contour in the form of a plane hollow body materializes. The wall intermediate element(s) is/are configured such that unfolding full-length is possible at least sectionwise. As a result of this, as described above, due to the folded wall intermediate elements, the desired volume dilation can occur substantially without material strain, in thus enabling the volume of the hollow body to be produced to be effectively increased, whilst retaining the full strain capacity of the material. Furthermore, the integral configuration of the wall intermediate element in the sheet metal blank ensures facilitated handling since there is no need to compose several individual sheet metal blanks at high expense and labor. In addition, jointing zones are avoided, as a result of which, the strength of the workpiece is increased.
To facilitate production of the intermediate product, it is good practice when this comprises at least one upper and one lower sheet metal section, arranged substantially superimposed. This simplifies joining and sealing the edge portions of the intermediate product as is necessary for effectively applying an internal high pressure for an internal high-pressure forming operation.
In certain applications, it is good practice when a fold of the wall intermediate element extends into a portion between the upper and the lower sheet metal section. With such a configuration it is possible, as already mentioned, to press the wall intermediate element by internal high pressure, as viewed in the cross-section of the intermediate product from the interior of the cavity, configured by the intermediate product, outwards and to unfold and stretch without material strain. In this arrangement, the wall intermediate element may be arranged folded, for example two, three or multi-ply between the two sheet metal sections.
As an alternative, it is good practice for uniform dilation of the intermediate product when at least one fold of the wall intermediate element is configured to protrude outwards beyond the remaining folds of the wall intermediate element, i.e. comprising a differing length. In this arrangement, it is mainly the upper and the lower sheet metal section that is subjected to pressure in the internal high-pressure forming method so that the wall intermediate element, as viewed cross-sectionally, moves from outwards inwardly and is thereby folded open into a wall section of the resulting hollow body.
To seal the intermediate product for later subjecting it to the internal high pressure, it is good practice when the sheet metal sections are joined at their faces at least sectionwise and/or to the faces of the wall intermediate element, to thus configure a flat hollow body closed off outwards.
It is likewise good practice for bridging an opening between the superimposed faces of the sheet metal sections, which may be provided due to the wall intermediate element arranged inbetween, to arrange a spacer which is joined to the faces of the sheet metal sections for closing off the face.
To facilitate inserting the intermediate product in a die for the internal high-pressure forming method it is furthermore good practice when the intermediate product is configured as a substantially flat hollow body.
For introducing the pressure medium into the intermediate product, it is preferred that the intermediate product comprises a blast socket or the like. The blast socket may be inserted at any desired location in the sheet metal blank or the wall intermediate element to facilitate filling the cavity surrounded by the intermediate product with the pressure medium.